One class of opto-electrical devices is those using an organic material for light emission or detection. The basic structure of these devices is a light emissive organic layer, for instance a film of a poly (p-phenylenevinylene) (“PPV”) or polyfluorene, sandwiched between a cathode for injecting negative charge carriers (electrons) and an anode for injecting positive charge carriers (holes) into the organic layer. The electrons and holes combine in the organic layer generating photons. In WO 90/13148 the organic light-emissive material is a polymer. In U.S. Pat. No. 4,539,507 the organic light-emissive material is of the class known as small molecule materials, such as (8-hydroxyquinoline) aluminium (“Alq3”). In a practical device one of the electrodes is transparent, to allow the photons to escape the device.
FIG. 1 illustrates the cross-sectional structure of a typical organic light-emissive device (“OLED”). The OLED is typically fabricated on a glass or plastic substrate 1 coated with a transparent first electrode 4 such as indium-tin-oxide (“ITO”). A layer of a thin film of at least one electroluminescent organic material 3 covers the first electrode. Finally, a cathode 2 covers the layer of electroluminescent organic material. The cathode is typically a metal or alloy and may comprise a single layer, such as aluminium, or a plurality of layers such as calcium and aluminium. Other layers can be added to the device, for example to improve charge injection from the electrodes to the electroluminescent material. For example, a hole injection layer such as poly(ethylene dioxythiophene)/polystyrene sulfonate (PEDOT-PSS) or polyaniline may be provided between the anode 4 and the electroluminescent material 3. When a voltage is applied between the electrodes from a power supply one of the electrodes acts as a cathode and the other as an anode.
For organic semiconductors important characteristics are the binding energies, measured with respect to the vacuum level of the electronic energy levels, particularly the “highest occupied molecular orbital” (HOMO) and the “lowest unoccupied molecular orbital” (LUMO) level. These can be estimated from measurements of photoemission and particularly measurements of the electrochemical potentials for oxidation and reduction. It is well understood in this field that such energies are affected by a number of factors, such as the local environment near an interface, and the point on the curve (peak) from which the value is determined. Accordingly, the use of such values is indicative rather than quantitative.
A focus in the field of polymer OLEDs is the development of full colour displays for which red, green and blue emissive materials are required. For commercial applications, it is desirable for the OLED to have a lifetime of several thousand hours (by “lifetime” is meant the time for the brightness of the OLED to fall from 100 cd/m2 to 50 cd/m2 when operated under DC drive at room temperature). One drawback with existing polymer OLED displays relevant to this development is the relatively short lifetime of blue emissive materials known to date. The lifetime of the emissive material may be extended by optimisation of the OLED architecture; for example lifetime of the red, green and blue materials may be dependant on the cathode being used. However, a polymer (in particular a blue electroluminescent polymer) that is inherently stable, i.e. one having a high lifetime in a variety of OLED architectures, is clearly desirable.
WO 00/55927 discloses a polymer of formula (a):
wherein w+x+y=1, w<0.5, 0≦x+y≦0.5 and n<2
This application does not disclose polymers comprising diphenylfluorene repeat units.
WO 00/22026 discloses polymers of formula (b):
wherein X is 97 or 99 and Y is 3 or 1 respectively. These polymers are disclosed for use in electroluminescent devices.
Other disclosures of electroluminescent polymers comprising diphenylfluorene repeat units include:    EP 1088875 which discloses a copolymer based on 9-(4-adamantylphenyl-10-phenylanthracene and 2-adamantylfluorene repeat units;    WO 99/20675 which discloses a 1:1 copolymer of 9,9-di-n-octylfluorene and 9,9-di(4-methoxyphenyl)fluorene.
It is an object of the present invention to provide a means for increasing the lifetime of polymers for use in an optical device above that of prior art polymers. It is a further object of the invention to provide a long-lived polymer for use in an optical device, particularly a long-lived blue electroluminescent material. It is a yet further object of the invention to provide a means for increasing the thermal stability of prior art polymers.